Detection of a centrifugal magnetosphere in one of the most massive stars in the ρOph star-forming cloud

Recent XMM-Newton observations of the B2 type star Oph A indicated a periodicity of 1.205 day, which was ascribed to rotational modulation. Since variability of X-ray emission in massive stars is frequently the signature of a magnetic field, we investigated whether the presence of a magnetic field can indeed be invoked to explain the observed X-ray peculiarity. Two FORS 2 spectropolarimetric observations in different rotation phases revealed the presence of a negative (B-zall = - 419 +/- 101 G) and positive (B-zall = 538 +/- 69 G) longitudinal magnetic field, respectively. We estimate a lower limit for the dipole strength as B-d = 1.9 +/- 0.2 kG. Our calculations of the Kepler and Alfven radii imply the presence of a centrifugally supported, magnetically confined plasma around Oph A. The study of the spectral variability indicates a behavior similar to that observed in typical magnetic early-type Bp stars.